The self-employment taxes on Line 57 of your Form 1040 would all belong to your wife.
If all other income amounts (other than your W-2 and your wife's self-employment income - Schedule C) are joint income you could calculate your respective income tax amounts as follows:
Add your W-2 income and your wife's self-employment income for the total earned income.
Divide your W-2 income by the total earned income and multiply by (the total taxes less the self-employment tax) would equal your share of the Federal taxes.
Divide your wife's self-employment income by the total earned income and multiply by (the total taxes less the self-employment tax), plus the Self-Employment tax equals your wife's share of the Federal taxes.
